Gas and steam turbines utilize servos for controlling actuators associated with various components of the turbines. The actuators typically move fuel valves, speed ratio valves, compressor vanes, and other mechanisms to control air and fuel flow in the turbine system. To control the position of the servo actuator, a precise and controlled amount of DC current (typically up to +/−200 mA) is passed through the actuator coil, and the current may be based in part on feedback from a transducer coupled to the mechanism or the actuator. Conventional servo controllers may provide the drive current for the actuators using linear buffers or linear amplifiers, which typically require bulky heat sinks to dissipate excess heat produced from the drive electronics.
In many turbines, the various valves and vanes may be controlled using hydraulic actuators. The position of the hydraulic actuators, valves, or vanes may be monitored and fed-back to the controller using transducers such as resolvers, linear variable differential transformers (LVDTs) or linear variable differential reluctance (LVDR) devices. Such devices are highly reliable in the harsh turbine environments, but they usually require AC excitation current for proper operation. The AC excitation current is typically provided by an excitation drive circuit with a linear output amplifier, which also can require a bulky heat sink to dissipate the excess heat produced by the drive electronics.
When turbines have a large number of valves, each with associated actuators and LVDTs, the turbine's servo controller may become excessively bulky due to the required number and size of heat sinks for the drive circuitry. Furthermore, when drive energy is converted to heat through the linear drive circuitry, the energy efficiency of the circuit is reduced, and the dissipated heat adds to the overall temperature of the control panel.